1. Field of the Invention
The present invention relates generally to an outboard motor attached to the stern of a boat and designed to produce a force required to thrust the boat, and in particular to an oil pan positioned below an engine of the outboard motor and a vertical wall disposed to be cooled down together with the oil pan and to form an exhaust passage such that the oil pan is protected from the heat of exhaust gas passing through the exhaust passage.
2. Description of the Related Art
Oil pans for holding engine oil are typically disposed below four-cycle engines of outboard motors. Such oil pans can be heated by the heat of exhaust gas passing through exhaust pipes connected to the engines. It is therefore necessary for the oil pans to be cooled down. A technique for protecting the oil pans from the heat of the exhaust gas is disclosed, for example, in Japanese Laid-Open Publication No. HEI-4-50097 entitled xe2x80x9cEXHAUST DEVICE FOR OUTBOARD MOTORxe2x80x9d.
The disclosed outboard motor includes an upper casing positioned below an engine. Within the upper casing, there is disposed an oil pan. The oil pan is channel-shaped in vertical cross-section. The oil pan has a recessed part. Adjacent the recessed part, there is formed a vertically extending water passage. An exhaust pipe vertically extends within the water passage. The exhaust pipe has its lower end portion located below the oil pan. The lower end portion of the exhaust pipe is in communication with a first exhaust expansion compartment. The first exhaust expansion compartment is in communication with a second exhaust expansion compartment. The second exhaust expansion compartment is provided above the first exhaust expansion compartment. Exhaust gas discharged from the engine flows through the exhaust pipe and the first exhaust expansion compartment up into the second exhaust expansion compartment. The upper casing has a lid disposed therewithin. The lid lies to separate the oil pan from the first exhaust expansion compartment, such that the heat of the exhaust gas in the first exhaust expansion compartment is prevented from affecting the oil pan. In other words, by virtue of the lid, the oil pan is cut off from the heat of the exhaust gas in the exhaust expansion compartment which would otherwise be transmitted thereto.
The exhaust pipe and an outer wall of the oil pan cooperate with each other to define a space therebetween. The water passage is formed along part of the periphery of the exhaust pipe. Part of cooling water for cooling the engine passes through the water passage to thereby cool the exhaust pipe.
To provide the thus arranged outboard motor, the lid for separating the first exhaust expansion compartment from the bottom of the oil pan positioned at great depths within the upper casing should be mounted in such a manner as to extend transversely of the upper casing.
The outboard motor is comprised of an engine case within which the engine having a vertically extending crankshaft is housed, the upper casing mounted to a lower part of the engine case, and a gear case connected to a lower part of the upper casing. The upper casing has the oil pan and the like surrounded by an upper inner surface thereof. The outboard motor thus arranged greatly extends in the vertical direction to thereby provide increased vertical dimension of the upper casing. The upper casing is therefore large in depth. It is thus difficult for the lid to be lain at a vertically central position within the upper casing of great depth.
Further, the aforementioned outboard motor is complicated in construction because: (1) The oil pan is disposed within the upper casing of great depth. (2) The longitudinally extending exhaust pipe is spaced from the outer wall of the oil pan. (3) The lid is positioned within the upper casing. (4) A passage for cooling water which has cooled the engine is provided outside a wall which forms the first exhaust expansion compartment. (5) The water passage is formed along the part of the periphery of the exhaust pipe. The outboard motor of such complicated construction undesirably requires the increased number of components.
An object of the present invention is to provide an outboard motor of simplified construction which can be readily made using the small number of components including an oil pan disposed to be unaffected by the heat of exhaust gas discharged from an engine of the outboard motor, so that oil held within the oil pan is prevented from increasing in temperature.
A further object of the present invention is to provide an outboard motor including an oil pan and a vertical wall disposed adjacent the oil pan to provide an exhaust passage, both of which are disposed to be cooled down simultaneously.
According to an aspect of the present invention, there is provided an outboard motor comprising: an engine having a vertically extending crankshaft; an oil pan disposed below a crankcase of said engine; a vertically extending drive shaft disposed to be driven by said crankshaft; a screw positioned at a lower end portion of said drive shaft, said screw extending perpendicularly to said drive shaft, said screw being disposed to be driven by said drive shaft through a gear transmission mechanism; a first case for housing therein said oil pan and an upper part of said drive shaft, said first case being disposed below said crankcase, said first case having a coupling surface coupled to said crankcase; a second case for housing therein a lower part of said drive shaft, said second case being disposed below said first case; a gear case for housing therein said gear transmission mechanism, said gear case being disposed below said second case; said first case having a vertical wall disposed therewithin to form an exhaust passage through which exhaust gas discharged from said engine passes, such that an outer wall of said oil pan is separated from said exhaust passage by said vertical wall; said second case having an upper edge portion coupled to a lower edge portion of said first case, said second case having a vertical wall disposed therewithin to form a cavity and an exhaust passage for discharging the exhaust gas into water.
The outboard motor of the present invention includes the first case and the second case. The first case has its lower edge portion coupled to an upper edge portion of the second case. Within the first case, there are housed the oil pan of large depth and the upper part of the drive shaft. By the vertical wall of the second case, the exhaust passage provided within the second case is separated from the cavity formed below the oil pan. With the thus arranged vertical wall, the least amount of the heat of exhaust gas passing through the exhaust passage affects the oil pan. Accordingly, engine oil held within the oil pan increases in temperature by the least amount.
The second case has the vertical wall corresponding in configuration to the vertical wall of the first case. The number of components required to provide the outboard motor can thus be reduced. The outboard motor of simple construction can be made using the reduced number of components. Such an outboard motor of simple construction is very easy to assemble or produce.
The engine oil is returned from the engine to the oil pan. The temperature of the engine oil thus returned is made relatively high by the heat of the engine. The engine oil of high temperature has been cooled down, to some extent, within the oil pan by the time it is resupplied to an engine block of the engine. The thus cooled engine oil can be therefore used to lubricate the engine. Consequently, it becomes possible to prevent the engine block and other components of the engine from increasing in temperature.
Further, since the exhaust gas is reduced in temperature, the engine makes a reduced noise when discharging the exhaust gas. Moreover, the heat of the exhaust gas passing through the exhaust passage affects the cases to a smaller extent.
In a preferred form of the present invention, said oil pan is formed integrally with said first case.
Accordingly, the oil pan and a portion of the first case for supporting the drive shaft is simple in construction. Since the oil pan is formed integrally with the first case, the first case can be readily made using the reduced number of components. Such a first case is coupled to the second case with reduced difficulty.
In a further preferred form of the present invention, the outboard motor includes a cooling water supply passage and water discharge passageways all of which are formed therein, said cooling water supply passage being positioned to supply said engine with cooling water for cooling said engine, said water discharge passageways being formed such that after cooling said engine, said cooling water flows out of said outboard motor by passing therethrough, at least one of said water discharge passageways being formed on a side of said outer wall of said oil pan provided inside said vertical wall.
Since the outer wall is disposed to be exposed to the cooling water, the cooling water passes over the outer wall, and thus effectively cools the oil held within the oil pan. With this arrangement, there is no need for such a particular cooling water passage structure as a water jacket for cooling the oil pan and the vertical wall. This simplifies the construction of the outboard motor. Thus, the outboard motor requires the reduced number of components.
In a still further preferred form of the present invention, the outboard motor includes a cooling water supply passage and water discharge passageways all of which are formed therein, said cooling water supply passage being positioned to supply said engine with cooling water for cooling said engine, said water discharge passageways being formed such that after cooling said engine, said cooling water flows out of said outboard motor by passing therethrough, at least one of said water discharge passageways being formed on a side of said exhaust passage provided outside said vertical wall.
The water discharge passageway is formed on the side of the exhaust passage. By flowing through the passageway, the cooling water lowers temperature in the exhaust passage. In addition, the cooling water cools the oil pan positioned adjacent the exhaust passage. The engine oil held in the oil pan can thus be effectively cooled. Accordingly, there is no need for such a particular cooling water passage structure as a water jacket for reducing temperature in the exhaust passage. This helps simplify the construction of the outboard motor. The outboard motor of simplified construction requires the reduced number of components.
With this arrangement, there can be minimized the extent to which the heat of the exhaust gas affects the engine disposed above the oil pan. There can be also minimized the extent to which temperature in an engine compartment is affected by the heat of the exhaust gas. In addition, it is unlikely that air to be taken into the engine increases in temperature. An output of the thus arranged outboard motor is not reduced when the engine operates at a high engine speed.